fix: correct LegacyDialogPrompt implementation
DefaultLegacyHooks: simplify plugin creation

The PluginService can be used to simplify creation of a plugin from a
PluginInfo.

Noted by ctrueden
DefaultLegacyService: update isLegacyMode check

It was not really accurate to use the legacyMode flag, now that we have
a LegacyUI.

When the LegacyUI is active we may be in "IJ2 mode" but still want the
legacyService.isLegacyMode method to return true.

So now we will check the IJ1Helper instead.
LegacyUI: implement show

Implementation of show that is very similar to that in
AbstractUserInterface.

The main difference is that if the obtained DisplayViewer is an
ImagePlusDisplayViewer, we can just call its view method and be done
without the need to create a display window, etc... as the ImagePlus
will handle it.
TEMP: open image test case

test an IJ2 plugin with an ImagePlus @Parameter, type=OUTPUT.